There have become known automatic metering apparatuses using a metering chamber formed by a space which is defined in an essentially vertical tube between a charge port and a discharge port by two slides laterally moveable in respect to longitudinal axis of the tube. For charging the upper slide is opened whilst the lower slide remains closed. After filling the chamber, the upper slide is closed whilst the lower slide is opened for discharging. Different volumes may be metered by insertion of space-filling bodies. In particular for materials having low density such as styropore or perlite, considerable metering inaccuracies result from changing flowability due to variations in environmental conditions. For the actuation of the slides, considerable force is necessary in order to overcome the friction whilst the space-filling bodies, arranged in the flowing path of the pourable material, obstruct the complete and fast discharge of the material out of the metering chamber.